In air cleaners such as used on internal combustion engines, there is typically a housing with a radially arranged snorkel through which unfiltered air is received and circulating about a toroidal-shaped air filter cartridge as shown in FIG. 1 of the accompanying drawing. The air filter is normally mounted in the housing between the inlet and the outlet and is spaced from the interior of the housing so as to define therewith an annular passage extending about the filter for delivering the unfiltered air from the inlet completely about the upstream side of the filter. Because of the radial inlet and the normally uniform cross section of the flow area about the filter, there results a non-uniform mass flow distribution as shown in FIG. 2 of the accompanying drawing whereas the ideal distribution would be uniform about the filter as shown by the dash line in this figure. Because of such non-uniform distribution, there is non-uniform or disproportionate use of the filter which reduces its effectiveness. Moreover, there results a disproportionate buildup of the majority of the dirt particles on the inlet and backside of the filter that can significantly shorten the useful life of the filter particularly the useful life of the filter particularly in those areas where there is a high degree of airborne foreign particles.